Optical coherence tomography is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). The principle of ocular imaging with OCT is based upon measuring the time-delay of the light reflected from each optical interface when a pencil of light (an “A-scan”) enters the eye. A series of A-scans across the structure permits a cross-sectional reconstruction of a plane through the anterior or posterior segment of the eye. This is known as a B-scan.
Speckle is problematic for OCT imaging as well as other imaging modalities including synthetic aperture radar (SAR), remote sensing, ultrasound, sonar, etc. In these fields, a substantial amount of signal processing research has been conducted to combat speckle noise, resulting in the development of powerful digital filters for reducing speckle while preserving edge sharpness. Speckle reduction in OCT has been addressed by modifying imaging configurations and through the use of signal processing.
Existing speckle reduction techniques are mainly based on frames using 1) filters such as Lee filter [1], Kuan filter [2], enhanced LEE filter [3]; 2) anisotropic diffusion such as proposed an anisotropic diffusion [4], speckle reduction anisotropic diffusion (SRAD) [5], detail preserving anisotropic diffusion [6] and orientated speckle reduction anisotropic diffusion [7]. These methods were first proposed for radar images and they are applicable for OCT images.
Topcon has developed DRI OCT-1, a swept source OCT for posterior imaging, utilizing a wavelength of 1,050 nm. It has a fast scanning speed of 100,000 A-scans/sec. Utilizing this 1,050 nm wavelength, DRI OCT-1 can penetrate deeper compared to the current conventional OCTs with wavelength in the 850 nm range. Visualizing ocular tissues such as choroid or even sclera, can be done within a very short time. Deep range imaging DRI OCT-1 displays detailed structures of not only retina but choroid and even sclera. With the capability of imaging deeper anatomic structures with less influence in scattering, DRI OCT-1 can visualize the entire tomogram with high sensitivity.
However, in the Topcon DRI OCT-1 swept source OCT, a single frame or slice of the image usually has poor quality due to large speckle noise. Accordingly, conventionally a moving average is computed. For example in the 3D mode with 256 frames, a moving average is computed from 3 to 5 neighboring frames in the DRI OCT-1. Although moving averaging has been shown to be able to reduce speckle noise, the reduction in speckle due to averaging over a small number of neighboring frames is limited. However, if more frames were included in the averaging, blur would appear as a visible artefact, especially in regions where the underlying retinal structures are changing rapidly.